It is well known in the art that wavelength division multiplexing (WDM) affords multiple channel communications over a single optical fiber link, thereby increasing transmission capacity without the need for higher speed components. For switching or networking applications, wavelength division multiplexing, moreover, permits optical routing of signals at different wavelengths to different destinations or stations.
Optical transmitters, however, used in wavelength division multiplexing must critically generate light at controlled wavelengths, either fixed or dynamically selectable. Such transmitters must restrict the wavelengths to preselected spaced values so that the optical signals do not interfere with each other. As such, wavelength division multiplexing systems benefit importantly from highly stable, wavelength selectable optical transmitters and, more particularly, from those producing modulated light with low chirp, i.e., low uncontrolled wavelength shifts.
Among the first attempts to provide such stable wavelength selectable transmitters were those using discrete fixed frequency lasers. An array of lasers, for example, comprising distributed Bragg reflector (DBR) lasers, is each integrated with an electroabsorption modulator, followed by an optical combiner and amplifier so as to provide multiple wavelength division channels over a single fiber link. An example of this approach is shown in the article by M. G. Young et al., entitled "A 16.times.1 WDM Transmitter With Integrated DBR Lasers and Electroabsorption Modulators," Paper No. IWA3, Technical Digest of 1993 Topical Meeting on Integrated Photonics Research, Palm Springs (1993) pp. 414-17. Unfortunately, such an approach requires a modulator for each laser, thereby requiring a complex electrical packaging to drive each modulator separately.
An alternative to the above approach has recently been developed wherein the cost and complexity make it more attractive for communication systems having a large number of optical channels, such as for local area networks and "fiber to the home" applications. This latter alternative monolithically integrates on a single substrate individually actuable lasers with an optical combiner. The output of the combiner containing the different wavelengths advantageously passes through only a single optical modulator, which is also likewise integrated on the same substrate. Of course, in this latter case, each wavelength is modulated on a time division basis with separate signals. See, U.S. Pat. No. 5,394,489, entitled "Wavelength Division Multiplexed Optical Communication Transmitters," which is incorporated herein by reference and commonly assigned.
Although optical transmitters based on the above latter approach perform acceptably, the material compatibility imposed by the monolithical integration may compromise the performance of the optical devices. Furthermore, and more importantly, such optical transmitters are substantially prone to having high chirp because of unwanted optical feedback. The extensive time resolved spectra (TRS) testing required and cost associated therewith to ensure that the chirp requirements are met make this approach unattractive for most optical communication systems.